Controlled thermal expansion of welds to enhance toughness

ABSTRACT

A method is provided for forming a metallic overlay having enhanced toughness. The metallic overlay may be a weld, a metallic coating, or similar application. The method includes applying a glass forming metallic alloy to a substrate while the alloy is in a molten or semi-molten state. At the interface of the metallic alloy overlay and the substrate the substrate metal becomes at least partially molten and combines with the alloy to form metallurgical bonds. When the metallic alloy cools it experiences a high relative degree of thermal contraction. The metallurgical bonds between the substrate and the alloy constrain the contraction of the alloy at the interface with the substrate. This results in the inducement of compressive stresses in the metallic alloy overlay. The induced compressive stresses inhibit the formation of cracks in the overlay and/or mitigation of the effects of any cracks in the overlay.

FIELD OF THE INVENTION

The present invention is directed at a toughening mechanism forimproving the properties and performance of weld-type overlays. Thetoughness of the weld-type overlay is improved by controlling thethermal contraction of the weld overlay during cooling. The increasedtoughness weld-type overlays of the present invention may be utilized inmany application including hardfacing, wear/overlay plate, as well asthe rebuild and repair of metal parts.

BACKGROUND OF THE INVENTION

Often with conventional materials, there is an inverse relationshipbetween hardness and toughness. Generally, as the hardness of thematerial increases there will be a corresponding, though not necessarilyproportional, decrease in the toughness of the material. On reason forthis inverse relationship is because the mechanism of dislocationmovement has a significant effect on both the hardness and the toughnessof a conventional material. When defects are introduced into a material,the defects may tie-up dislocations, thereby preventing the materialfrom yielding. This mechanism makes the material both harder andstronger. Conversely, removing defects from a material allowsdislocations to move freely on their slip plane and slip directionproducing a greater degree of ductility. From a general standpoint,resistance to cracking (i.e. toughness) will be determined by thematerial's ductility because stress concentrations in front of a cracktip will create a plastic zone which blunts the crack tip, reducing thestress concentration factor, thus preventing growth of the crack.

While the thermal spray coatings industry is a mature industry and theapplication of a high performance coatings have long been used todramatically improve the lifetime of a part, there are many military andindustrial applications for which a thermal spray coatings approach isnot sufficient to solve wear problems. Problematic applications ofteninvolve heavy loads, high stress point loads, heavy impact, and gougingabrasion of the coated part. Additionally, while thermal spray may beused for limited cases in the rebuild and repair of parts, weld ontechniques will generally be necessary.

Accordingly, it is an object of the present invention to provide themost efficient balance of hardness and toughness in a metallic coating,so that, in a given application, both parameters may be uniquelyoptimized to improve the lifetime of a part to both wear and impact typephenomena.

SUMMARY OF THE INVENTION

In a first embodiment the present invention is directed at a method forforming a metallic overlay comprising supplying a metal substrate with athermal expansion coefficient “X”, supplying a metal alloy which has athermal expansion coefficient “Y”, wherein Y>X, melting said metal alloyand applying said metallic alloy to said metal substrate to form analloy/substrate interface, forming metallurgical bonds between saidmetallic alloy and said substrate at said alloy/substrate interface, andcausing said alloy to shrink while said alloy is constrained at saidalloy/substrate interface thereby developing a residual compressivestress in said metallic alloy.

In a second embodiment the present invention is directed at a method forforming a metallic overlay comprising supplying a metal substrate with athermal expansion coefficient “X”, supplying a metal alloy which has athermal expansion coefficient “Y”, wherein Y>X and wherein said metalalloy has a yield strength “Z”, melting said metal alloy and applyingsaid metallic alloy to said metal substrate to form an alloy/substrateinterface, forming metallurgical bonds between said metallic alloy andsaid substrate at said alloy/substrate interface, and causing said alloyto shrink while said alloy is constrained at said alloy/substrateinterface thereby developing a residual compressive stress in saidmetallic alloy, wherein said compressive stress does not exceed theyield strength “Z”.

In a third embodiment the present invention is directed at a method forforming a metallic overlay comprising supplying a metal substrate with athermal expansion coefficient “X”, supplying a metal alloy which has athermal expansion coefficient “Y”, wherein Y>X and wherein said metalalloy has a yield strength “Z”, melting said metal alloy and applyingsaid metallic alloy to said metal substrate to form an alloy/substrateinterface, forming metallurgical bonds between said metallic alloy andsaid substrate at said alloy/substrate interface, and causing said alloyto shrink while said alloy is constrained at said alloy/substrateinterface thereby developing a residual compressive stress in saidmetallic alloy, wherein said compressive stress does not exceed theyield strength “Z” and wherein said metal alloy has a hardness ofgreater than about 850 kg/mm².

In yet another embodiment the present invention is directed at a methodfor forming a metallic overlay comprising supplying a metal substrate,supplying a metal alloy, melting said metal alloy and applying saidmetallic alloy to said metal substrate to form an alloy/substrateinterface, forming metallurgical bonds between said metallic alloy andsaid substrate at said alloy/substrate interface, causing said alloy tocool to provide said alloy with a fracture toughness greater than 200MPa m^(1/2) and a hardness greater than 5 GPa.

BRIEF DESCRIPTION OF THE DRAWINGS

An understanding of the invention herein, including objects, features,and advantages is provided by a description of specific exemplaryembodiments thereof, which description should be read and understood inconjunction with the accompanying figures, wherein

FIG. 1 shows photographs of an arc melted ingot of Alloy A before (onthe left) and after (on the right) being hit with a moderate blow from aball peen hammer;

FIG. 2 is a plot of thermal expansion for high velocity oxy-fuel couponsof Alloy A, Alloy B, Alloy C, and Alloy D;

FIG. 3 is a plot of thermal expansion of a coupon of Alloy A up to atemperature of 1000° C., both of an as-sprayed sample and of a fullycrystallized sample; and

FIG. 4 is a plot of toughness versus hardness showing thehardness/toughness of Alloy C compared to published results forexemplary iron alloys, aluminum alloys, nickel alloys, carbides,nitrides, and oxides.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention is a method of providing a metallic overlay to asubstrate that has improved toughness. The method involves a mechanismof developing compressive stresses in the metal material after cooling(residual compressive stress). The induced residual compressive stressdue to shrinkage both prevents cracks from forming and acts to close thetip of any cracks that form. By preventing or mitigating cracks in themetallic overlay it is possible to significantly reduce the stressconcentration factor experienced at the crack tips.

As used herein, the tem “weld overlay” refers to a metallic materialthat has been applied to a substrate in an at least partially moltenstate. Furthermore, the term weld overlay contemplates a fused interfacebetween the metallic material and the substrate, such that there is atleast partial metallurgical bonding between the metallic material andthe substrate. Metallurgical bonding includes a chemical bondinginteraction forming metallic-type chemical bonds between the metallicmaterial and the substrate.

Accordingly, a weld overlay may include, but is not limited to metallicmaterial applied in a welding process, a thermal spray metal coating, inwhich a molten or semi-molten metal is sprayed onto a substrate, or afused coating in which a metallic coating is heated and caused to fuseto the substrate. Various other coating types and methods will beunderstood in which a metallic material is at least partially fused to asubstrate from a molten or semi-molten state thereby formingmetallurgical bonds with the substrate.

Similarly, it should be understood that weld material refers to anymetallic material that is applied in a manner contemplated hereinaboveand/or applied forming metallurgical bonds with a substrate or baseconsistent with the present invention. Generally these metallicmaterials may be classified as glass forming metallic alloys. Mostespecially, suitable metallic glasses may be iron based glass formingalloys. These suitable alloys exhibit high hardness and yield strengthand will have the ability to form glasses at high cooling rates.However, actual glass formation is not a priori since there are caseswhere the glass forming region is just missed during solidification buta high level of undercooling is achieved. This undercooling can providea large driving force to aid in the rapid transformation to a nanoscalestructure. Exemplary compositions could include any base metal withsufficiently high glass forming ability and sufficiently high thermalexpansion.

The present invention recognizes that when, e.g., glass forming alloysare welded they can be made to experience greater contraction on coolingas compared to a conventional steel substrate. During welding, intricatemixing occurs between the weld material and the base metal, and a full,or at least partial, metallurgical bond may be formed from the liquidmelt and may be subsequently maintained during cooling. As the weldmaterial cools it shrinks in all directions but it is constrained in atleast one direction by the intimate contact/metallurgical bonding withthe base metal. Therefore, as the weld deposit is cooled it contracts toa higher degree than the base metal/substrate and, therefore, solidifiesinto a state having high compressive residual stress. This favorableresidual stress prevents cracks from forming and/or propagating in theweld material. In addition, these built up and retained compressivestresses inhibit the formation of cracks in the weld material, andthereby increase the toughness of the weld material.

The development of residual stress as disclosed herein has not beenobserved to occur in conventional metals to the same degree. Whenconventional weld material solidifies, if there are large differences incoefficient of thermal expansion between the weld material and thesubstrate large localized stresses may arise. If these localizedstresses exceed the yield strength of the weld material plastic flow ofthe material may occur which acts to release or relieve the residualstress. If the plasticity or total elongation of the weld material isexceeded in a localized area, crack formation may be initiated.

In addition to being able to form high residual compressive stress, thepresent invention utilizes the unique ability of glass forming alloys toretain such residual stress upon solidification. One aspect of this isthe high yield strengths found in this class of materials. For example,measured yield strengths for iron based glass forming alloys can be ashigh as 3000 MPa at room temperature and as high as 1800 MPa at 700° C.By comparison, it should be noted that “Ultra High Strength Steels” maygenerally have room temperature yield strengths in the 1380-1520 MParange. At 700° C. the above alloy exhibits a higher yield strength thanso called ultra high strength steels present at room temperature. Thehigher yield strengths of iron based glasses support the understandingthat high residual compressive stress is maintained in the welddeposits, but the stress does not exceed the yield strength of the weldmaterial, i.e. the stress interacts in the elastic range of thematerial. Utilizing these findings, coatings, welds, etc. can beprovided in which both plastic deformation and cracking phenomena may beavoided and high residual compressive stress is maintained.

According to the present invention a metallic glass may be deposited ona substrate, for example as a weld or thermal spray coating. Using suchtechniques, the metallic glass is deposited in a molten or semi-moltenstate. The heat of the metallic glass being deposited and/or additionalprocessing conditions may cause at least a portion of the surface of thesubstrate to achieve a molten or semi-molten state as well. Desirablythe metallic glass being deposited will at least partially fuse to thesubstrate, forming metallurgical bonds between the metallic glass andthe substrate. As the metallic glass cools from the as-applied molten orsemi-molten state it experiences thermal shrinkage that is relativelyhigh. The key is that the thermal expansion of the referenced metallicglass has a higher thermal expansion coefficient than the base substratematerial, preferably at least about 15.0% higher. The metallurgicalbonding between the substrate and the metallic glass restricts theshrinkage of the metallic glass along the interface thereof. As aresult, high compressive stresses are induced in the metallic glass. Theoverall effect may be somewhat analogous to shot peening or hammerforging, although the mechanism are distinguishable.

As alluded to above, the present invention is susceptible to use in avariety of approaches involving weld processing, or similar processinginvolving the formation of metallurgical bonds between, preferably, aglass forming alloy and a substrate. Suitable processes may includePlasma Transferred Arc (PTA) welding, Metal Inert Gas (MIG) welding,Laser Engineered Net Shape (LENS), Shielded Metal Arc Welding (SMAW),Powder Welding, and Gas Tungsten Arc Welding (GTAW). These exemplaryprocesses may utilize a powder feedstock, a flexible wire feedstock, ora solid wire feedstock. However, the form of the feedstock or the exactprocess used is not a limiting aspect for this invention.

The invention herein accordingly pertains to improved toughness of aweld overlay. In that regard, it is worth noting that the hardness ofthe weld overlay will be dependant on a variety of factors including themicrostructure scale, the level of supersaturation of alloying elements,and resistance of specific grain boundary pairs to resist grain boundarysliding and grain boundary rotation.

EXPERIMENTAL EXAMPLES

Four experimental alloys were produced having the compositions detailedin Table 1 using generally conventional alloying techniques. Themetallic alloys were provided as cored wire having a diameter of{fraction (1/16)}″. The cored wire of the various alloys were processedusing a MIG (metal inert gas) welding apparatus operating at 32V and250A with a welding gas shield consisting of 98% Ar-2% O₂ to producesample hardfacing deposits which were deposited onto various plaincarbon and alloy steel substrates. TABLE 1 Alloy Designations andCompositions. Alloys Compositions (Wt %) Alloy A 78.1 Fe, 9.2 Cr, 4.3Mo, 4.1 B, 1.3 C, 0.6 Si, and 2.4 Al Alloy B 65.9 Fe, 25.3 Cr, 1.0 Mo,1.8 W, 3.5 B, 1.2 C, 0.5 Si, 0.8 Mn Alloy C 64.9 Fe, 26.0 Cr, 1.0 Mo,1.4 W, 3.6 B, 1.2 C, 1.0 Si, 0.8 Mn Alloy D 68.0 Fe, 23.2 Cr, 1.2 Mo,1.5 W, 3.6 B, 0.9 C, 0.7 Si, 0.8 Mn

As a first experimental test, the hardness of welds produced using AlloyB and Alloy C were determined using Rockwell C hardness testing. Weldsproduced using wire stock from Alloy B and Alloy C were found to haveunexpectedly high hardnesses of R_(c)=62 and R_(c)=65, respectively.Additionally, Alloy C and Alloy D were tested to determine the Vickershardness. As with the Rockwell C hardness of Alloy B and Alloy C, theVickers hardness of weld deposits formed from Alloy C and Alloy D provedunexpectedly high, exhibiting values of 950 kg/mm² and of 1100 kg/mm²respectively.

The toughness of the alloys was experimentally evaluated using a hammeror hammer and chisel to apply direct blows to the substrate that hadbeen hardfaced with weld deposits of the experimental alloys. Generally,it had previously been observed that alloys having the compositionsdetailed in Table 1 have very low toughness in ingot form. For example,one moderate blow from a ball peen hammer may often cause the ingots tocrack apart. Such a typical result is shown in FIG. 1, in which an ingotof Alloy A, formed by arc-melting, is shown before (on the left) andafter (on the right) being stricken with a moderate blow from a ballpeen hammer. In contrast to the expected result, weld deposits of theexperimental alloys exhibit much higher toughness. In experimentalevaluation, repeated hammer strikes to a weld-deposit hardface coatingof the experimental alloys failed to produce any observable cracking ofthe weld deposits. Furthermore, repeated (>50) blows with a hammer andchisel resulted in only very small amounts of material being removedfrom the weld, at most much less than one gram. During testing, 4different tool steel chisels were flattened and repeatedly sharpened andthen reflattened as a result of striking the weld material.

In addition to the hammer and chisel tests, which were remarkable, asample cross section of an Alloy C weld deposit was tested for toughnessusing the Palmqvist Technique. During the Palmqvist testing, theindentation load was initially set to be 2 Kg, and was subsequentlyincreased up to a 90 Kg load. No cracking was observed in the welddeposit even up to the maximum testing load of 90 Kg. Since no crackingwas observed in the Alloy weld, it was not possible to obtain anumerical measure of toughness using the Palmqvist technique. However,it may still be possible to use the Palmqvist technique to estimate alower limit to the fracture toughness by assuming a mean radial cracklength on the general order of 10⁻⁷ m to 10⁻⁸ m, which is below theresolution of an optical microscope (10⁻⁶ m). Using this assumption, theestimated lower limit of fracture toughness of the Alloy C weld depositwould be in the range of 22 to 70 MPam^(1/2).

By way of comparison, relevant literature, for example D. K. Shetty, I.G. Wright, P. N. Mincer and A. H. Clauer, J. Mater. Sci. 20, 1873,(1985), has revealed that cemented tungsten carbide begins crackingduring Palmqvist testing at much smaller indentations loads,approximately on the order of 2.5 Kg. Also, the literature indicatesthat the expected mean radial crack length for cemented tungstencarbides at an applied 90 Kg load could be estimated to be approximately1000 microns. It should be noted that the Palmqvist method of measuringFracture Toughness is well established in the weld on hardfacing andsintered carbide industries and is the industry standard to measuretoughness. Based on previous studies, the Palmqvist toughness can becorrelated fairly accurately to the plain strain fracture toughness(K_(1c)). See, for example, D. K. Shetty, I. G. Wright, P. N. Mincer andA. H. Clauer, J. Mater. Sci. 20, 1873, (1985); and G. R. Anstis, P.Chantikui, B. R. Lawn and D. B. Marshall, J. Am Ceram. Soc. 64, 533,(1981).

Referring to FIG. 4 the toughness versus hardness for a variety ofmaterials including iron alloys, aluminum alloys, nickel alloys,carbides, nitrides, and oxides is shown. As shown, the general inverserelationship between hardness and toughness is observed. On the plot, itcan be seen that the Alloy C weld (indicated as DAR), occupies a newmaterial regime, with novel combinations of toughness and hardness. Ascan be seen in FIG. 4, Alloy C not only exhibits uniquely high fracturetoughness, but the high fracture toughness is achieved without anattendant decrease in hardness. Table 2 through Table 10 below presentthe data of FIG. 4 in tabular format. TABLE 2 Hardness and FractureToughness for Selected Oxides. Hardness Fracture Oxide Compound (GPa)(MPa(m)1/2) Al2O3 26 2 Al2O3 19 6 Al2O3 23 4 MgO 8 2.5 MgAlO4 18 1.9MgAlO4 14 2.4 Mullite 15 3 ThO2 10 1.6 Y2O3 8 1.5 ZrO2 15 3 ZrO2 12 3.6ZrO2 7.4 9 TiO2 7.4 1.4 TiO2 10.5 1.9

TABLE 3 Hardness and Fracture Toughness for Selected Carbides. HardnessFracture Carbides (GPa) (MPa(m)1/2) SiC 26 6 SiC 36 3 SiC 27 4 SiC 19.34 SiC 21.1 3.1 TiC 28 3 TiC 16 5 BC 72.2 6

TABLE 4 Hardness and Fracture Toughness for Selected Nitrides. HardnessFracture Nitrides (GPa) (MPa(m)1/2) Si3N4 30 3 Si3N4 17 10 Si3N4 14.14.9

TABLE 5 Hardness and Fracture Toughness for Selected Tungsten Carbides.Hardness Fracture WC—Co (GPa) (MPa(m)1/2) WC—Co 16.72 9.4 WC—Co 16.339.3 WC—Co 14.93 9.9 WC—Co 11.77 13.1 WC—Co 16.87 7.7 WC—Co 15.06 8.1WC—Co 16.75 9.6 WC—Co 19.61 8.9 WC—Co 14.09 9.5 WC—Co 14.27 9.3 WC—Co15.3 8.2 WC—Co 13.3 10 WC—Co 15.7 7.6 WC—Co 17.46 6.4 WC—Co 19.84 5.1WC—Co 13.29 9.9 WC—Co 16.84 6.9 WC—Co 15.58 7.8 WC—Co 12.74 11.6 WC—Co12.33 12.2 WC—Co 11.37 14.5 WC—Co 11.46 14.1 WC—Co 10.84 15.5 WC—Co10.92 15.2 WC—Co 11.86 13.3 WC—Co 11.96 12.9 WC—Co 11.045 14.5 WC—Co10.09 17.1 WC—Co 13.2 16

TABLE 6 Hardness and Fracture Toughness for Selected Titanium Alloys.Hardness Fracture Ti Alloy (GPa) (MPa(m)1/2) Ti—5Al—2.5Sn 3.136 76.93Ti—6Al—2Cb—1Ta—1Mo 2.94 98.91 Ti—8Al—1Mo—1V 3.43 65.94 Ti—6Al—4V 3.62665.94 Ti—6Al—6V—2Sn 3.332 60.445 Ti—6Al—6V—2Sn 4.312 24.178Ti—6Al2Sn4Zr—6Mo 3.43 36.267 Ti—6Al2Sn4Zr—6Mo 3.92 24.178Ti—13V—11Cr—3Al 3.332 87.92 Ti—13V—11Cr—3Al 4.214 38.465

TABLE 7 Hardness and Fracture Toughness for Selected Aluminum Alloys.Hardness Fracture Al Alloys (GPa) (MPa(m)1/2) 1.323 23.2 0.931 29.11.2054 32.3 1.47 22.5 2014 1.323 18.683 2024 1.176 28.574 2219 1.27436.267 5086 0.7056 49.455 6061 0.931 28.574 7075 1.47 20.881

TABLE 8 Hardness and Fracture Toughness for Selected Steel Alloys.Hardness Fracture Steel Alloy (GPa) (MPa(m)1/2) 3.724 109.9 4.214 74.7325.39 52.752 4.9 48.356 4.606 71.435 2.2442 64.841 4.802 71.435

TABLE 9 Hardness and Fracture Toughness for Selected Nickel Alloys.Hardness Fracture Ni Alloy (GPa) (MPa(m)1/2) 5.096 38.465 5.488 29.6735.194 76.93 5.488 60.445 5.096 46.158 4.508 65.94 5.292 32.97 4.60665.94 4.214 82.425 3.528 131.88 4.704 49.455 5.39 74.732 5.39 79.1285.8016 49.455 0.441 155 0.4704 120 0.4998 80 0.4214 125

TABLE 10 Hardness and Fracture Toughness for Selected DAR Alloys.Hardness Fracture DAR Alloy (GPa) (MPa(m)1/2) DAR 8.3 22.28413 8.370.46859 8.3 222.8413 8.3 704.6859

Additional testing of the experimental alloys included differentialscanning calorimetry (DSC) of Alloy B. The DSC analysis indicated thatthe alloy contained at least a small fraction of glass. The presence ofthe glass fraction was indicated by a peak at approximately 615° C.,which is the temperature of the metallic glass transition for an alloyof the tested composition. Both Alloy C and Alloy D were also designedto have an increased glass forming ability compared to Alloy B.

The experimental examples discussed above indicate that the MIGweld-deposited alloys consistent with the present invention have a highdegree of toughness and a high level of hardness. At the time of filing,it is believed that this toughness is related to the differentialthermal expansion of the weld-deposited material as compared to thesubstrate on which the material is deposited. This theory was based ontesting of the thermal expansion of selected iron based glass formingalloys which were measured over the temperature range of 20-1000° C. Thetests of thermal expansion were conducted using a Theta IndustriesDilamatic II dilatometer on coupons of the alloys produced by highvelocity oxy-fuel spraying. The experimentally determined thermalexpansion of the alloys versus temperature is shown in FIG. 2. In thisplot, it is noted that the reduction in slope found in each alloy wasverified to be the result of the volume reduction which occurs when theglass crystallizes as shown in FIG. 3. It was noted that the beginningof the reduction in slope for each alloy corresponds to the glasscrystallization temperatures for each respective alloy.

Referring to FIG. 3, a plot of the thermal expansion of the Alloy Aversus temperature is shown, both for an as-sprayed test specimen andfor a specimen that had been completely crystallized prior to testing.It can be seen from this plot that the completely crystallized specimendid not experience a reduction in expansion with increasing temperaturebecause the specimen was free of glass.

Based on the above experiments, it was found that the glass formingsteel alloys exhibit relatively high thermal expansions. The testresults of thermal expansion coefficients for the experimental alloyscompared to several commercial steel alloys are listed in Table 11. Itcan be seen that these specialized iron based glass forming alloys havemuch higher thermal expansion coefficients than many conventional ironbased alloys, as reported in William D. Callister, Jr., MaterialsScience and Engineering, John Wiley & Sons, New York, 1994. TABLE 11Coefficient of Thermal Expansion for Various Alloys (100 to 500° C.).Alloy CTE (ppm/° C.) Alloy A 14.34 Alloy B 14.84 Alloy C 14.73 Alloy D14.75 Iron 11.8 1020 steel 11.7 1080 steel 11.0 410 stainless steel 9.9

1. A method for forming a metallic overlay comprising: supplying a metalsubstrate with a thermal expansion coefficient “X”; supplying a metalalloy which has a thermal expansion coefficient “Y”, wherein Y>X;melting said metal alloy and applying said metallic alloy to said metalsubstrate to form an alloy/substrate interface; forming metallurgicalbonds between said metallic alloy and said substrate at saidalloy/substrate interface; and causing said alloy to shrink while saidalloy is constrained at said alloy/substrate interface therebydeveloping a residual compressive stress in said metallic alloy.
 2. Themethod of claim 1 wherein said alloy is comprised of a mixture of Fe,Cr, Mo, W, B, C, Si and Mn.
 3. The method of claim 2, wherein Fe ispresent at levels above 50.0 wt %.
 4. The method of claim 2, wherein Fe,Cr, Mo, and W comprise at least 90 wt % of said mixture.
 5. The methodof claim 1 wherein Fe and Cr comprise at least 90 wt % of said mixture,and Cr is present at levels of about 1.0 wt. %, and Mo is present atlevels of about 1.0-2.0 wt %.
 6. The method of claim 1 wherein Fe and Crcomprise at least 90 wt. % of said mixture, and Cr is present at levelsof about 1.0 wt. %, and Mo is present at levels of about 1.0-2.0 wt. %,and W is present at levels of about 3.0-4.0 wt %, B is present at levelsof about 1.0-2.0 wt %, C is present at levels of about 0.1-1.0 wt %, Siis present at levels of 0.1-1.0 wt % and Mn is present at levels of0.1-1.0 wt %.
 7. The method according to claim 2 wherein said metallicalloy has a composition of about 65.9 wt % Fe, 25.3 wt % Cr, 1.0 wt %Mo, 1.8 wt % W, 3.5 wt % B, 1.2 wt % C, 0.5 wt % Si, 0.8 wt % Mn.
 8. Themethod according to claim 2 wherein said metallic alloy has acomposition of 64.9 wt % Fe, 26.0 wt % Cr, 1.0 wt % Mo, 1.4 wt % W, 3.6wt % B, 1.2 wt % C, 1.0 wt % Si, 0.8 wt % Mn.
 9. The method according toclaim 1 wherein said metallic alloy has a composition of 68.0 wt % Fe,23.2 wt % Cr, 1.2 wt % Mo, 1.5 wt % W, 3.6 wt % B, 0.9 wt % C, 0.7 wt %Si, 0.8 wt % Mn.
 10. The method according to claim 1 wherein applyingsaid metallic alloy comprises welding.
 11. The method according to claim1 wherein applying said metallic alloy comprises thermal spray coating.12. The method according to claim 1 wherein said metallic alloy has acoefficient of thermal expansion greater than 15% of that of the basesubstrate.
 13. The method according to claim 1 wherein said iron basedmetallic alloy has a coefficient of thermal expansion in the range of 12to 17 ppm/° C.
 14. A method for forming a metallic overlay comprising:supplying a metal substrate with a thermal expansion coefficient “X”;supplying a metal alloy which has a thermal expansion coefficient “Y”,wherein Y>X and wherein said metal alloy has a yield strength “Z”;melting said metal alloy and applying said metallic alloy to said metalsubstrate to form an alloy/substrate interface; forming metallurgicalbonds between said metallic alloy and said substrate at saidalloy/substrate interface; and causing said alloy to shrink while saidalloy is constrained at said alloy/substrate interface therebydeveloping a residual compressive stress in said metallic alloy, whereinsaid compressive stress does not exceed the yield strength “Z”.
 15. Themethod of claim 14 wherein said compressive yield strength is greaterthan about 1520 MPa at room temperature.
 16. A method for forming ametallic overlay comprising: supplying a metal substrate with a thermalexpansion coefficient “X”; supplying a metal alloy which has a thermalexpansion coefficient “Y”, wherein Y>X and wherein said metal alloy hasa yield strength “Z”; melting said metal alloy and applying saidmetallic alloy to said metal substrate to form an alloy/substrateinterface; forming metallurgical bonds between said metallic alloy andsaid substrate at said alloy/substrate interface; and causing said alloyto shrink while said alloy is constrained at said alloy/substrateinterface thereby developing a residual compressive stress in saidmetallic alloy, wherein said compressive stress does not exceed theyield strength “Z” and wherein said metal alloy has a hardness ofgreater than about 850 kg/mm².
 17. A method for forming a metallicoverlay comprising: supplying a metal substrate; supplying a metalalloy; melting said metal alloy and applying said metallic alloy to saidmetal substrate to form an alloy/substrate interface; formingmetallurgical bonds between said metallic alloy and said substrate atsaid alloy/substrate interface; and causing said alloy to cool toprovide said alloy with a fracture toughness greater than 200 MPam^(1/2) and a hardness greater than 5 GPa.